Researchers can now say definitively that Mars once
supported a watery environment, but whether the red planet could have ever
supported life is still far from certain.

The
success of NASA's Mars rover Opportunity in finding tell-tale
signs of past water at its Meridiani Planum landing site has left some
researchers believing the region could have once been a habitable, albeit
still hostile, environment.

"We
can say 'yeah, it was probably a habitable environment, but was it an
environment in which life could have arisen,'" said Cornell University's Steve
Squyres, principal science investigator for the Mars rover mission, in a
telephone interview. "That's a good question."

While
Opportunity has not found any signs of life since landing at Meridiani in
January, knowing water was once abundant there makes it a good candidate for
future study.

"In
everything we know about life on Earth, there is no known example without liquid
water," Squyres said. "That's the reason the search for water was so important
at Mars."

Squyres' Mars water
research appears today in the journal Science alongside a series of other
studies from data taken during Opportunity's first 90 days on Mars. Since then,
the hardy rover has tallied about 300 days exploring Mars while its robotic twin
Spirit has spent an even longer time trekking across Gusev Crater on the other
side of the planet.

Potential
challenges for potential life

If some sort of life did take advantage of Mars'
watery past, it may have had a hard time of
it. Researchers are not sure how widespread
water was and how long it may have persisted, which could have put
restraints on the development of microorganisms
and
other creatures.

"One of the questions we
don't have and answer for is how the water was situated," Squyres added.
"Was it sort of little localized puddles or continuous over
kilometers?"

Squyres said it
is also unknown whether any of the water bodies that once littered
Meridiani were exposed to the atmosphere. They may have
been covered by an ice shell, with water oozing up out of the
ground, he added.

Researchers were
able to determine -- with the aid of Opportunity's science package -- that liquid water
did flow on surface of Mars in the past, its currents etching ripples in
stone as a calling card.

"Now we have a fairly
specific indication of what the water medium for life may have been
like," said Jeff Kargel, a researcher with the U.S. Geological Survey in
Flagstaff, Arizona who commented on Opportunity studies in today's Science issue. "We don't know if there
was life on Mars, but it certainly is more of a plausible
idea."

Mineral deposits left
behind as Martian water evaporated include substances like hematite and
jarosite, which
indicate a once wet, acidic and salty
waterscape.

"[Mars water] must have
been very acidic to support the formation of minerals like jarosite, at least as
acidic as tomato juice," Kargel told SPACE.com.

On Earth, such
acidic or saline-rich waters - such as the wine-colored Rio Tinto
basin in Spain.

"The stuff is just teeming
with life," Squyres said of Rio Tinto water. "But the organisms there didn't
develop first in that environment...they evolved to inhabit it."

Whether similar life could
have originated in the water environment of Mars' past is unknown, Squyres
added.

Developing a water
story

Understanding the composition of
past water on Mars has helped researchers shape their view of the planet's
watery history.

"There's much more to it that just
saying water was there," Squyres said. "We've made some significant steps in
characterizing what the water environment was like."

Kargel
and Squyres agree that going back to Meridiani Planum with a future mission
and returning a physical sample to Earth could shed more light on the region's
aqueous past, as well as the possibility of life there. If any organisms ever
lived in Mars water, the nature of the rocks at Meridiani Planum would have
been adept at preserving chemical and physical fossils of them, they
said.

"I'd
love to get a hunk of some of this stuff and get it into the best laboratory on
Earth," Squyres said.

But
some researchers urge caution. In his Science
commentary, Kargel stated that future Mars missions may need to assume life exists first in
order to effectively prevent contamination to both Mars and Earth in future
sample return or human missions.

"The idea here is to get
the bio-burden down," Squyres said. "I think as responsible explorers we have an
obligation to keep contamination to a minimum."